Internal combustion engines operate by the controlled combustion of hydrocarbon fuels and produce exhaust gases containing complete combustion products such as carbon dioxide (CO2) and water (H2O), and incomplete combustion products such as carbon monoxide (CO) and unburned hydrocarbons (HC). Further, as a result of the very high temperatures produced by the burning of the hydrocarbon fuels, thermal fixation of nitrogen in the air results in the detrimental formation of nitrogen oxide compounds (NOx). Certain undesirable components of the exhaust, including hydrocarbons, soot particulates, CO, and NOx, must be controlled to meet government emissions regulations.
The quantities of pollutants generated by incomplete combustion varies with operating conditions of the engine but are influenced predominantly by the air-to-fuel ratio in the combustion cylinder. Conditions conducive to reducing carbon monoxide and unburned hydrocarbons, i.e., a fuel mixture just lean of stoichiometric and high combustion temperatures, cause an increased formation of NOx, and conditions conducive to reducing the formation of NOx, i.e., rich fuel mixture and low combustion temperatures, cause an increase in carbon monoxide and unburned hydrocarbons in the exhaust gases. As a result, within the region of stable operation of the internal combustion engine, significant amounts of CO, HC and NOx are emitted from the engine.
It is obviously desirable to reduce the emissions of a warmed up engine operating at high flow steady state conditions. However it is also very important to reduce the cold start emissions of the engine, which can be achieved by employing a fast light-off exhaust catalyst. When the vehicle is started, the catalyst can be heated to light-off temperature, using, for example, exhaust heating, injection of reformate, and heating of reformate using an in-exhaust burner.
Reformates are hydrogen-enriched fuels that can be produced from a variety of sources, including gasoline, diesel, and other liquid or gaseous fuels. On-board reformers for producing hydrogen-enriched reformate fuels are described in, for example, U.S. Pat. Nos. 6,655,130 and 6,832,473 and U.S. Patent Appl. Publ. Nos. 2004/0146458 and 2005/0022450, the disclosures of which are incorporated herein by reference.
The vehicle exhaust aftertreatment system of the present invention, which is particularly suitable for use with a diesel engine, includes an exhaust catalyst that comprises a small volume NOx adsorber coupled with a small volume oxidation catalyst. This configuration of the exhaust catalyst enables it to be quickly heated to the temperature where NOx storage begins to occur, typically above 150° C., and then to light-off temperature, where the catalyst is able to convert the stored NOx to N2. This rapid storage and light-off also allows the amount of included precious metal to be reduced.